1. Field of the Invention
This invention relates to a platinum catalyst and more particularly to a platinum catalyst supported on carbon particles.
2. Description of the Prior Art
Platinum is a well-known catalyst used in electrochemical cells. Electrode performance in a cell is directly related to the amount of surface area of platinum which can be reached by the various reacting species within the cell. This fact, coupled with the high cost of platinum, has resulted in considerable effort to get platinum into a usable form which has maximum surface area per unit weight of platinum. The basic approach has been, and still is, to put the platinum on the surface of suitable particles called supports. Carbon particles and graphite particles are common platinum supports in the fuel cell art. Several known techniques exist for depositing small platinum particles on such supports. For example, the support can be dispersed in an aqueous solution of chloroplatinic acid, dried, and exposed to hydrogen. Some other techniques are described in U.S. Pat. Nos. 3,857,737 to Kemp et al, 3,440,107 to Barber, 3,470,019 to Steele, and 3,532,556 to Steele.
By these techniques platinum crystals may be dispersed on the surfaces of the support particles so as to provide a high surface area of platinum.
When carbon supported platinum is used at temperatures of greater than 100.degree. C. in the presence of a liquid (or at higher temperatures in the presence of a gas) it has been found to lose surface area. This loss of surface area is particularly pronounced in an acid fuel cell environment, such as in fuel cells using phosphoric acid as the electrolyte, which operate at temperatures anywhere from 120.degree. C. and higher. The loss in surface area is dramatic during the first few hours of cell operation, but it continues at a slow but steady rate for a considerable period thereafter. A loss in cell performance is directly attributable to this loss in platinum surface area.
One method for reducing this loss of surface area is described in commonly owned U.S. Pat. No. 4,028,274 to Harold R. Kunz. In that invention the surfaces of graphitized carbon support particles were oxidized in the presence of a metal oxidizing catalyst to form pits in the surfaces of the particles. The metal oxidizing catalyst was then removed and the platinum was deposited on the oxidized particles. Based on the theory that during use of the catalyst the platinum crystallites migrate over the surface of the carbon and combine with other platinum crystallites (i.e., recrystallizing) thereby losing surface area, it was felt that the pits in the surface of the support material would hold the platinum crystallites more securely in place thereby reducing migration and loss of surface area. While this method was an improvement over the prior art, it was not totally satisfactory and continued efforts have resulted in the improved method of the present invention.